RFC 1195 [IS-IS for IP Networks] is an interior gateway protocol utilized to determine the topology and routing within a network with the network divided between level 1 areas and a level 2 domain connecting the level 1 areas. IS-IS works in conjunction with a link state protocol that advertises the adjacencies of each of the nodes in an area or domain of an IS-IS network. The IS-IS protocol utilizes a route computation algorithm such as shortest path first (SPF) algorithm to calculate routes from each network device in an IS-IS area or domain to each other network device in that area or domain as well as routes (for level 1 routers) to the border routers that enable data to be forward to destinations outside of the IS-IS network area. Each network device in the IS-IS area or domain constructs a routing information base in which the next hop to each other destination network device in the IS-IS area or domain is recorded to be utilized to forward data packets.
When an adjacent router or the link to that adjacent router designated as the next hop for a route to a given destination network device fails, traffic disruption will occur until the network re-converges (i.e., recalculates paths to each of the network devices in the IS-IS area or domain). This disruption of traffic can be prevented by Loop Free Alternatives (LFA) technology in principle as specified in RFC 5286. LFAs are alternate routes to each network device that can be utilized in the case that a next hop to a given destination is not available.
In many IS-IS network areas or domains network devices are connected with a set of adjacent routers through multiple parallel links. These links can have different characteristics including differences in bandwidth, latency and other metrics. Other metrics related to routers can include load, provisioning and similar dynamic traffic conditions. Such metrics alone or in combination can be the basis for calculating a ‘cost’ for each link that can be utilized in routing computations including the SPF computation.